Within internal combustion engines working under strong load and using fuels which contain sulfur, sometimes in relatively high amounts, serious problems arise, especially with regard to lubrication. The strong acidity produced by the combustion of the fuel induces corrosion in the engine and an intense oxidation of the lubricating oil which leads to a formation of varnish and sludge.
One preferred way to counteract these detrimental effects is to incorporate an alkaline earth metal sulfonate having a high degree of alkalinity into the lubricating oil. The detergent effect of alkaline earth metal sulfonates is well known in the art.
The basic content or degree of "overbasing" is generally expressed in terms of the alkalinity value (AV) of the material and is measured by titration (ASTM D-664 and preferably D-2896) in terms of mg KOH/g. Overbased sulfonates can be made having alkalinity values of from 1-500 mg KOH/g or more depending on the method of preparation.
Overbasing is in general accomplished by one of two methods. The first method involves the neutral detergent sulfonate solubilizing extra base into the oil solution followed by filtration. This is accomplished by boiling a mixture of neutral sulfonate, base and oil. Products having an alkalinity value of about 20 are produced. See, for example, "Dispersions of Insoluble Carbonates in Oils" by Ulric B. Bray et al., Ind. Eng. Chem. Vol. 14, No. 4, 1975. A second method of overbasing is to treat the inorganic oxide, hydroxide, etc., with an acidic gas, preferably carbon dioxide, to form in situ a very finely dispersed metal carbonate. Very high alkalinity values can be obtained this way ranging from 300-500 AV or more. See, for example, U.S. Pat. No. 4,086,170.
It has also been found that a variety of supplemental compounds can be added during the various overbasing processes to aid in the incorporation of base. Representative of such compositions are dihydric alcohols containing up to 6 carbon atoms such as ethylene glycol, propylene glycol, 1,2 butanediol and the like (taught in U.S. Pat. No. 2,676,925), alcohols including glycerol, pentaerythritol and the like (taught in U.S. Pat. No. 3,242,079), and alcohols such as methanol, ethanol, sorbitol, pentaerythritol and the like (taught in U.S. Pat. No. 3,256,186 and 2,861,951).